Synnefo » snf-ganeti

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Ganeti Bulk Create

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Current state and shortcomings

==============================

Creation of instances happens a lot. A fair load is done by just

creating instances and due to bad allocation shifting them around later

again. Additionally, if you turn up a new cluster you already know a

bunch of instances, which need to exists on the cluster. Doing this

one-by-one is not only cumbersome but might also fail, due to lack of

resources or lead to badly balanced clusters.

Since the early Ganeti 2.0 alpha version there is a ``gnt-instance

batch-create`` command to allocate a bunch of instances based on a json

file. This feature, however, doesn't take any advantages of iallocator

and submits jobs in a serialized manner.

Proposed changes

----------------

To overcome this shortcoming we would extend the current iallocator

interface to allow bulk requests. On the Ganeti side, a new opcode is

introduced to handle the bulk creation and returning the resulting

placement from the IAllocator_.

Problems

--------

Due to the design of chained jobs, we can guarantee, that with the state

at which the ``multi-alloc`` opcode is run, all of the instances will

fit (or all won't). But we can't guarantee that once the instance

creation requests were submit, no other jobs have sneaked in between.

This might still lead to failing jobs because the resources have changed

in the meantime.

Implementation

==============

IAllocator

----------

A new additional ``type`` will be added called ``multi-allocate`` to

distinguish between normal and bulk operation. For the bulk operation

the ``request`` will be a finite list of request dicts.

If ``multi-allocate`` is declared, ``request`` must exist and is a list

of ``request`` dicts as described in :doc:`Operation specific input

<iallocator>`. The ``result`` then is a list of instance name and node

placements in the order of the ``request`` field.

In addition, the old ``allocate`` request type will be deprecated and at

latest in Ganeti 2.8 incooperated into this new request. Current code

will need slight adaption to work with the new request. This needs

careful testing.

OpInstanceBulkAdd

-----------------

We add a new opcode ``OpInstanceBulkAdd``. It receives a list of

``OpInstanceCreate`` on the ``instances`` field. This is done to make

sure, that these two loosely coupled opcodes do not get out of sync. On

the RAPI side, however, this just is a list of instance create

definitions. And the client is adapted accordingly.

The opcode itself does some sanity checks on the instance creation

opcodes which includes:

* ``mode`` is not set

* ``pnode`` and ``snodes`` is not set

* ``iallocator`` is not set

Any of the above error will be aborted with ``OpPrereqError``. Once the

list has been verified it is handed to the ``iallocator`` as described

in IAllocator_. Upon success we then return the result of the

IAllocator_ call.

At this point the current instance allocation would work with the

resources available on the cluster as perceived upon

``OpInstanceBulkAdd`` invocation. However, there might be corner cases

where this is not true as described in Problems_.

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